SU-F-J-60: Impact of DIR Method On Treatment Dose Wrapping.

PURPOSE: To investigate clinical relevant discrepancy between doses wrapped by pure image and biomechanical model based deformable registration (DIR).

METHODS: 12 patients, each with a CT pair, were included (5 H&N, 5 Prostate and 2 Lung). A research DIR tool (ADMRIE) was utilized for image based DIR (IMG-DIR). To assure organ matching, contour constrain was applied for prostate patients. Tetrahedron meshes were generated for organs (parotid, bladder, rectum and lung). Deformable vector fields (DVF) from IMG-DIR were interpolated to the surface node of meshes as boundary condition. Biomechanical models using finite element modeling (FEM) were generated by assigning organ specific material properties. The models were then input into a FEM tool (ABAQUS) to calculate internal deformation (FEM-DIR). The output volume node displacements were then interpolated to image grids to get refined DVF. The IMRT treatment doses were wrapped by both DVFs to pre-treatment CTs. DVF vector distance (DVF-VD) was calculated on each organ. Dose parameters were calculated for wrapped doses and normalized to pretreatment plan. Gamma passing rate (GPR) was calculated with IMG-DIR dose as reference. Correlation was evaluated between parotid shrinkage and DVF-VD /dose-discrepancy.

RESULTS: H&N:parotid volume with DVF-VD (>1.5mm) was 6.5±4.7%. The normalized mean dose difference (NMDD) of IMG-DIR and FEM-DIR was -0.8±1.5%, with range (-4.7% to 1.5%). 2mm/2% GPR was 99.0±1.4%. Moderate correlation was found between parotid shrinkage and DVF-VD (R=0.61)/NMDD (R=0.68). Prostate:bladder had a NMDD of -9.9±9.7%, with FEM-DIR wrapped dose systematically higher. Only minor deviation was observed for rectum NMDD (0.5±1.1%). 3mm/3% GPR of bladder and rectum were 81.9±12.0% and 93.1±4.3%, respectively. One of lung patients had 3.9%NMDD and 3mm/3%GPR of 95.2% inside lung.

CONCLUSION: Impact of DIR methods on treatment dose wrapping is patient and organ specific. Generally, bigger organ with larger volume variation leads to greater dose wrapping uncertainty.Acknowledgement:Elekta research grant support. This work was supported by research funding from Elekta.